Shear blade

ABSTRACT

Shear blade for use in band mills used in lumber processing, the shear blade adapted to restrict propellering of lumber. Leading edge of shear blade is tight, being at higher stress than remaining portions of blade, tightness attained by using one or more of three approaches. First approach is to apply asymmetrical load to the blade, load being applied forward of center line of blade. Second approach is to make trailing edge longer than leading edge, and third approach is to roll-tension shear blade.

United States Patent Allen 1 SHEAR BLADE [72] lnventor: Francis E.Allen, 956 Beaumont Drive, North Vancouver, British Columbia, Canada[22] Filed: July 7, 1970 [21] Appl. No.: 52,943

[52] US. Cl. ..143/22, 143/17, 83/201.l5, 143/ 159 S [51] Int. Cl...B27b 15/08 [58] Field ofSearch ..l43/l59 8,156,159,159 S, 143/22, 133C, l9, 17, 157, 17 R, 19 R;

[ 56] References Cited UNITED STATES PATENTS 3,516,458 6/1970 Hedrei..l43/2 2 3,456,539

7/1969 Amada ..83/201.l5

[ 51 3,693,675 [451 Sept. 26, 1972 1,790,282 l/l93l Phillips ..l43/l33 R592,936 11/1897 Pryibil 143/156 R 142,036 8/1873 Merritt 143/156 R1,723,389 3/1929 Thiel ..l43/l 56 R Primary Examiner-Donald R. SchranAttorney-Brian J. Wood [5 7] ABSTRACT 9 Claims, 8 Drawing Figures itsPATENTEDsP2s 1972 SHEET 2 or 3 Francis E. Allen,

PATENTEDSEPZB I972 SHEET 3 [IF 3 Francis E. Allen,

Inventor SHEAR BLADE 1 Field of the Invention The invention relates toband mills as used in saw mills and in particular to twin band millsused to produce flitches or to rip cants.

2. Prior Art A fiitch is a slab of lumber having two opposite parallelsawn sides and waney edges. A cant is a length of lumber having twopairs of sawn opposite parallel sides.

Twin band mills have been used for many years, a difficulty withexisting twin band mills being in maintaining a flat kerf accuratewithin required limits, and in maintaining the sawn sides parallel. Thekerf can undulate or twist from causes such as, knots, and variation inhardness or friction, in the log. Unequal cutting forces on each bladeproduce an unbalanced couple tending to produce rotation of thelog as ittravels through themill, such rotation being referred to aspropellering. Propellering tends to produce helical surfaces, rotationof the log being limited by interference of sawn faces of the log onside walls of the band saws. Such interference consumes power, reducingeffectiveness of cutting, and heats the saw. Heating changes tension andchange in tension tends to cause the saw to wander which, as is wellknown in the art, is both disadvantageous and dangerous.

One attempt to reduce the above difficulty is described in Canadian Pat.No. 804,359, Constantin Hedrei inventor and granted to Forano Limitee in1969. The patent teaches a guide within the kerf mounted behind andco-planer with a cutting portion of a saw blade of a twin band mill,that is to say the log encounters the guide after it has been cut by thesaw.

Cutting portion, as used herein, means the one or more teeth which, at aparticular instant, are actually cutting. Stiffness of the guide isincreased by applying a tensile load, the guide being shaped so that aleading edge nose portion is maintained under a tension lower than thatina remaining portion of the guide. The lower tension results in a slackleading edge adapted to follow slight curves or twists in the kerf, andthus tends to produce sawn sides which are not parallel. The guide isthicker than the kerf and tends to wedge it open, thus relievingfriction on the side walls of the saw.

The patent above also teaches inclination of the leading edge of theguide to compensate somewhat for cutting forces produced by the saw asit cuts the log, the cutting forces acting downwardly on the log andforcing the log onto the tracks or conveyor. Inclination of the leadingedge reduces load on the conveyor or tracks which may reduce wear on theconveyor and power consumed in feeding logs through the saw.

SUMMARY OF THE INVENTION The present invention contemplates a thin shearblade or guide with a stiff leading edge. The thin blade does not wedgethe kerf open, thus frictional forces on sides of the shear blade arelow. The stiff leading edge maintains alignment of the log with the saw,maintain- The shear blade has a thickness less than the width of thekerf and has a leading edge and a trailing edge separated by a centerportion. The leading edge is maintained at a higher tensile stress thanthe center portion and trailing edge, a leading edge so stressed beingknown in the trade as a tight leading edge.

There are at least three approaches to obtaining a tight leading edge.

A first approach to tighten the leading edge is by applying asymmetricalloading to the shear blade, by displacing at least one point ofapplication of the load to anchor plates of the shear blade forward of acenter line of the shear blade so that more load is carried by theleading edge of the shear blade than by the trailing edge.

A second approach is to apply biased loading to the shear blade byhaving the trailing edge longer than and inclined to the leading edge,the leading edge being disposed nonnalto direction of travel of lumber.

A third approach is to roll-tension the shear blade, as in a saw bladeof a band mill, so that tire portions at leading and trailing edges ofthe shear blade are produced and, when the saw blade is under tension,the tire portions are maintained at a higher stress than the centerportion.

One, two or all three of the above approaches can be used to attain atight leading edge for the shear blade.

A detailed description following, related to drawings, givesexemplification of the invention which, however, is capable ofexpression in structure other than that particularly described andillustrated.

DESCRIPTION OF THE DRAWINGS FIG. 1 is a perspective of a twin band millequipped with shear blades according to the invention,

FIG. 2 is a fragment of a portion of one shear blade in position behinda cutting portion of a band saw,

FIG. 3 is a plan of a log being cut by saws of a twin band mill equippedwith two shear blades,

FIG. 4 is an end elevation of a flitch being cut by a twin re-saw, there-saw being equipped with one shear blade,

FIGS. 2 4 are simplified and diagrammatic,

FIG. 5 is a fragmented side elevation of the shear blade when unloaded,anchor means mounting the blade being shown partly sectioned,

FIG. 6 is a simplified section of the shear blade on 6-6 of FIG. 5 someparts not being shown,

FIG. 7 is a fragmented section of a leading edge of the shear blade on7-7 of FIG. 5,

FIG. 8 is a fragmented detail section of an upper end of the shear bladeon line 8-8 of FIG. 5, showing in addition means of mounting andloading.

DETAILED DISCLOSURE FIG. 1

Band mills, l0 and 11, have upper wheels 12 and 13 on which ban saws land 15 run. Shear blades 16 and 17 according to the invention are shownaligned with the saw 14 by a distance 25, in the order of 2 or 3 g teeth21 of the saw 14 travelling in a direction shown by an arrow 26 theteeth actually cutting being a cutting portion, 24, as defined. Upwardlyprojecting spikes 27' of the conveyor 23 hold the log passing it throughthe saw. Cutting forces on the log are several orders of magnitudegreater than feed forces on the log produced by feeding it through thesaw. The log is thus forced onto the spikes, the spikes sinking into thelog. A conveyor fitted with knife-type flights can be used in lieu of aspiked conveyor.

With reference to FIG. 3, the log 22 passing through the twin band millson the spiked conveyor 23 is cut producing a flitch 29.2, and two slabs29.1 and 29.3. The saw blade 14, having a thickness, or gauge, 32 hasteeth 21 swaged at cutting edges as shown in profile and designated21.1, the teeth producing a kerf 30 having a width 31,. which width isgreater than the thickness of the saw 14. The shear blade 16 has athickness, or gauge, 33 less than the saw gauge 32. Hereinafter planesof the shear blade and the cutting portion of the saw blade refer toplanes midway between parallel side faces of the blades. The shear bladeis disposed symmetrically behind the saw blade, with the saw and theblade planes co-planar.

Ideally the shear bladedoes not wedge open the kerf,

the blade being co-planar with and positioned closely,

behind the saw, with clearance provided in the kerf on either side ofthe shear blade. In practice slight rotations and transverse movements,of the log, or closing ofthe kerf, produce binding on one or bothsurfaces of the shear blade. This restricts further transverse movementor rotation of the log, such restriction being a prime function of theshear blade. Restriction above produces in the logs cut by bandmillsequipped with shear blades kerfs within closer limits than if shearblades were eliminated. The shear blade serves as a means within thekerf of the log to maintain the kerf within a plane of the cuttingprotion of the saw blade, so that movement of the logs through the sawis essentially axial translation with negligible rotation of the log.

FIG. 4

When re-sawing a flitch, as shown in FIG. 4, only one shear blade isused inthe twin band mill.

The flitch 40 carried on a slat bed 42, is cut by twin band saws, planesof the cutting portions of blades being designated 44 and 45, the bladesnot being shown. A crowder roll 47 having an axis of rotation 48 forcesthe flitch 40 against a line bar 49, which is parallel to the direction.of feed of flitches through the saw. The crowder roll maintains a datumface 40.1 of the flitch 40 against the line bar,'thus ensuring that akerf flat within required limits is made through the flitch. When usedas above, only one shear blade (not shown) is fitted behind the saw 44,which saw is nearer the crowder roll than the saw '45. The one shearblade serves to prevent a flitch, produced from the larger flitch 40,from rotating'axially or wandering laterally and possible interferingwith other flitches produced concurrently. The crowder roll ispositioned adjacent to the cutting portion of the saw 44 and rotates tofeed the flitch through the saws. A cant can be cut as above, using oneshear blade only,and satisfactory results are obtained.

In FIG. 4, the flitch or cant is fed through the saw on a moving slatbed, thus the spiked conveyor chain of FIG. 3 is eliminated and the slatbed is used in lieu. Other means of feeding lumber through the saw areknown.

It was stated previously with reference to FIG. 3 that the gauge 33 ofthe shear blade is less than the gauge 32 of the saw. This is usual butin some circumstances the shear blade gauge can be greater than the sawgauge. Best results are obtained when the shear blade gauge is less thanthe width 31 of the kerf 30, however in some circumstances satisfactoryresults can be obtained whenthe shear blade gauge approaches or evenexceeds somewhat the width 31 of the kerf. In such circumstances,excessive binding of the kerf on the shear blade may result and adequatelubrication is provided to reduce build-up of resin etc.

FIG. 5 I I The shear blade 16 is a quadrilateral having an upper end 61having a width 62, a lower end 63 having a width 64, a leadingedge 66and a trailing edge 67, leading and trailing edges being defined withreference to the direction 19 indicating motion of the lumber past theshear blade.

A mid position 68 of the end 61 and a mid position 69 of the end 63 arejoined by a center line 71. The upper end 61 is held in an upper anchorplate 73 and the lower end 63 is held in a lower anchor plate 74, slotsin the plates accepting the ends of the shear blades. A plurality ofdowels 75 and 76 (shown in end elevation) in the anchor plates 73 and 74locate the ends of the shear blade and are fitted through location holesin the plate and the shear'blade after fitting as above. The anchorplate 73 is hinged at an upper pivot pin having an axis 79, and theanchor plate 74 is hinged at a lower pivot pin having an axis 81. A line83 joins the axes 79 and 81, which line represents a mean line of actionand direction of tension forces applied on the blade, the line beingforward of the centerline 71. A column (not shown) supporting the shearblade has a vertical center line designated 85 which center line, asseen in FIG. 5, is parallel to the leading edge 66. The leading edge 66is at an angle 87 to an upper edge at the upper end 61 and an angle 88to a lower edge at the lower end 63, both the angles being 90. Since thedirection 19 is at right angles to the leading edge 66, there is novertical reaction component tending to lift the log off the chainconveyor. The width 64 is greater than the width 62, so the center line71 of the shear blade is inclined at an angle 91 to the vertical centerline 85 of the column and the trailing edge is longer thanthe leadingedge. The axis 79 of the upper pivot point is on the center line 85 andthe axis 81 of the lower pivot point is displaced behind the center line85 by a distance 89 so as to incline the line 83 to the center line 85at an angle 93 as shown.

The pivot pin at the axis 81 is secured to the column of the band millso that the assembly 74 is free to rotate about the pin in a limitedrange. Load is applied to the pin at 79 in a direction 95 parallel tothe center line 85 to tension the shear blade to increase effectivestiffness of the shear blade, to be explained.

A central concept of the invention is to ensure that the leading edge ofthe shear blade is stiffer than a remaining portion of shear blade.Stifiness above is attained by applying load to the shear blade in sucha manner that the leading edge is at a higher tensile stress than theremaining portion of the shear blade without subjecting the shear bladeto a material bending moment. Such a distribution of stress in theleading edge of the shear blade is referred to as producing a tightleading edge, three approaches being used to attain this tightness. Atight leading edge has been defined as maintaining the leading edge at ahigher tensile stress that the center portion and trailing edge of theshear blade when the blade is under load in the mill and is attained byusing at least one of three approaches, namely asymmetrical loading,biased loading, or roll-tensioning. Insufficient tightness of theleading edge can result in the leading edge going slack because of localheating, such slackness reducing effectiveness of the shear blade.

Asymmetrical Loading (FIG. 5)

Asymmetrical loading is attained by positioning the axes 79 and 81 ofthe pivots forward of the center line 71 of the shear plate, additionalstress above symmetrical loading of the shear blade being applied todowels adjacent the leading edge'of the blade, thus tightening theleading edge of the shear blade. In this instance both axes aredisplaced forward of the center line 71 to tighten the leading edge,however, displacement of only one forward of the center line may besufiicient in some cases; typical displacement being in the order ofone-quarter of an inch for a blade having a width from about 7 to 10inches.

Such forward displacement of at least one point of application of loadto the anchor plates of the shear blade results in a gradual decrease instress from the leading edge to the trailing edge across a typicalsection of the shear blade.

Biased Loading (FIG. 5)

Biased loading of the shear blade is attained when the trailing edge islonger than the leading edge. A blade as above, when subjected to atensile load applied generally parallel to the leading edge results inthe leading edge being at a higher stress than the trailing edge, suchstress resulting from biased loading. Upper and lower edges of the bladecan be parallel as shown, differences in widths 61 and 63 being about 2inches for a shear blade of 4 foot length with a mean width of about 10inches. Upper and lower edges of the quadrilateral need not be parallelas shown, provided that the leading edge is shorter than the trailingedge and is disposed generally parallel to the load applied to the shearblade.

The two approaches above result in the leading edge of the shear bladehaving a tensile stress higher than the trailing edge or center portion.Material for the shear blade permits a relatively wide variation ofstress from the leading edge'to the trailing edge whilst maintaining thestress in the leading edge below the yield point of the material andmaintaining a reasonably stiff trailing edge. Common saw steel has beenused for the shear blade and is satisfactory.

Roll-tensioning (FIG. 6)

As stated above, the shear blade is made from saw steel and isroll-tensioned similarly to a band saw, such roll-tensioning beingachieved by stretching the center portion of the shear blade by plasticdeformation, leaving tire portions adjacent outer edges such that, in anunloaded condition when the blade hangs from an upper or lower edge in afree state, the shear blade has .a section as seen in FIG-6. A centerportion 98 of the ing and trailing edges and defines, in part, a datumplane of the shear blade. The bow is maximum adjacent the center line71, which maximum is a dimension 101 being of the order of thirtythousandths of an inch for a shear blade having a mean width of about 10inches.

One measure of the bow is that the distance 101 would be maintainedalong the total length of the shear blade if the shear blade were curvedto form an arc of a circle having a diameter of about 35 feet, with acenter of the circle on a side of the datum plane 99 remote from thecenter portions 98.

A tire portion 102 having a width 103 is adjacent to the leading edge 66and a further tire portion 104 having a width 105 is adjacent thetrailing edge 67. The widths 103 and 105 are of the order ofthree-quarters of an inch and are essentially co-planar with the datumplane above.

One result of the roll-tensioning is that, considering the centerportion 98 of the shear blade separated from the tire portion 102 and104 and placed in a free state on a flat surface, length of the centerportion would be marginally greater than length of the tire portions.Rolltensioning can be effected by hammering, or other means known to thetrade.

When the shear blade lies in a relaxed condition on a flat surface, thetire portions are effectively under low tension and the center portionis under low compression. When the shear blade hangs freely in a relaxedcondition, the tire portion remains relatively flat while the centerportion bows away from the datum plane of the blade, the center portionbeing referred to as the dished portion. When sufficient load is appliedto the shear blade, the dished portion tends to become aligned with thedatum plane 99, thus the shear blade approaches a flat sheet. The tireportions which were already under some tension before the blade wasloaded, are thus more highly stressed than the center portion. Thus rolltensioning' results in tire portions having a higher stress than thecenter portion and is the third approach that ensures that the edgeportions are at a higher stress than the center portion.

All three approaches above ensure that under normal operatingconditions, at no time whilst the blade is loaded will the leading ortrailing edges go slack due to expansion produced by heat generated bymovement of the logs past the shear blade.

When using two or more of these approaches, contributions of stress dueto each of the approaches depends on load applied to the shear blade,dimensions of the shear blade, degree of roll tensioning, anddisplacement and inclination of the line 933 from the center line 71 ofthe shear blade. The degree of roll tensioning can be defined asdifference in stress between the tire portions and the dished potion ofthe blade when the blade is in the relaxed condition on a flat surface,and in the particular shear blade described, leading edge tightness isattributed mainly to the roll tensioning.

A significant factor in roll tensioning is choice of a suitable steelthat responds adequately to roll tension-.

ing. Saw steel is suitable, although other steels can be heat treatedand/or cold worked for roll tensioning. A band saw steel having anultimate tensile strength range from about 200,000 to 210,000 pounds persquare inch and a yield strength of l80,000 to 190,000 poundsper squareinch gives satisfactory results in practice. Dif ficulty has beenexperienced in attempting to use mild steel and stainless steel.

In essence, a leading edge having a stress higher than the remainder ofthe shear blade ensures that, in spite of heating due to friction, theleading edge remains tight so that the log or. flitch is guided by thetight leading edge 66 in the kerf which, in most operating conditions,is essentially coplanar with the saw blade.

As stated before, sufficient leading edge tightness may be obtained byusing only one or two of the three approaches above, required degree oftightness being dependent on operating conditions andexpected'temperature rises due to heat generated by friction. Heat canbe removed from the shear blade by providing coolant/lubricant on bothsurfaces of the blade, rate of cooling of the blade being controlledgenerally by volume flow. Friction between the shear blade and the logcan be decreased by using a low friction coating on surfaces of theshearblade, a suitable coating being Teflon-S, a registered trade markof Du Pont. FIG. 7 I

In H6. 7, the leading edge 66 as shown has a chamfer 106 on one side1070f the shear blade which chamfer is at an angle 108 to surfaces ofthe shear blade, the angle 108 being of the order of about thirtydegrees. The side 107 of the shear blade faces the band mill column,disposition of the chamfer 106 being such as to ease access of the shearblade into the kerf. F K]. 8 a

A portion of the column of the band mill is designated 110 in FIG. 8,the column having the center line 85 (shown only in FIG. The sectionshown in FIG. 7 is from a section plane containing the center line 85. Asupport bracket 112 is secured at an inner end 113 to the portion of thecolumn 110, the bracket hav ing a vertical bore 114 as shown. Anysufficiently rigid portion of the band mill having abore aligned withthe axis 83 will suffice, eliminating the bracket 112. A pivot clevis116 is secured by a pin 118 to the upper anchor plate 73, the pin beingconcentric with the axis 79.. The line 83 joining axes of the pivotpoints passes as a central axis through the shear blade 14 as shown. Theshear blade is held in a slot 119 at a lower end of the plate 73 by thedowels 75 (in broken outline in FIG. 8). The plate 73fis freeto pivot ina limited manner on the pin 1 18 so as to ensure that negligible bendingmoment is applied to the shear blade by loading of the blade.

A stud 121 extends vertically from the pivot clevis 1 l6 co-axially withthe axis 83, and extends through the bore 114. An upper end of the stud121 has a stack of load-indicating washers 124 interposed between a nut125 and a locking nut 126. The load-indicating washers 124 can be of adished. type that deflect under load, measurement of deflection givingan indication of load on the washers, which is effectively load on theshear blade. Suitable washers are Solon spring washers or Bellevillespring washers obtainable from ordinary trade sources.

With the ends of the shear blade dowelled in the anchor plates and theplates mounted on the pivot pins, the stud 121 is fitted in the bore114, and the washers 124 and nuts 125, 126 are threaded onto the stud.The

nut 125 is tightened until desired deflection of the stack of washers isattained, thus permitting application and estimation of load on theblade. Typical load applied to an approximately ten inch wide shearblade of 15 gauge saw steel [0.072 inches thick] is about 10,000 pounds.

Other means of applying and measuring load applied to the shear bladecan be used, for example hydraulic ram or lever means, thus eliminatingthe washers 124.

The description of the blade 16 above applies to the shear blade 17above. The description relating to FIGS. 1 through 4'is use of shearblade with twin band mills. Cutting operations using a single band millonly benefit by using a shear blade, description of FIGS. 5 through 8applying to both single and twin band mils.

I claim:

l. A bandmill (10) having a band saw (14) adapted to cut a log (22)moving in a direction on a conveyor (23), the saw having a thickness(32) and a cutting portion (24), the cutting portion having a planeparallel with the direction of movement of logs, the saw being adaptedto cut a kerf (30) in the log, the kerf having a width (31 ,l, thebandmill including:

a. a shear blade (16) mounted coplanar with and spaced behind thecutting portion of the saw, the shear blade having leading and trailingedges (66, 67) a center portion (98), and a thickness (33) less than thewidth of the kerf,

b. tensioning means to tension the shear blade so that the leading edgeis at a higher tensile stress than the center portion of the shear bladeadapted so that movement of the logs through the saw is essentiallyaxial translation with negligible rotation of the log.

- 2. A bandmill as defined in claim 1 in which the tensioning means totension the shear blade includes:

7 c. tire portions (102, 104) at the leading and trailing edges of theshear blade, the tire portions being produced by roll tensioning,adapted so that, when the shear blade is subjected to tensile stress,the tire portions are at a higher tensile stress than the center portion(98) of the shear blade.

3. A bandmill as defined in claim 1 in which the shear blade hasmid-points .(68, 69) at upper and lower ends (61, 63), and thetensioning means to tension the shear blade includes:

(1. pivots at the upper and lower ends of the shear blade the pivotshaving axes (79, 81), at least one axis of a pivot being displacedforward of a midpoint of one of the ends of the shear blade, the pivotpoints being adapted so that a tensile load applied between the pivotpoints is applied to the shear blade, adapted so that, when a tensileload is applied to the pivots, the leading edge of the shear blade is ata higher tensile. stress than the trailing edge and center portion.

. A bandmill as defined in claim 1 in which:

e. the shear blade is a quadrilateral with the trailing edge longer thanthe leading edge,so that when the tensile load is applied to the shearblade the leading edge is at a higher tensile stress than the trailingedge.

5. A bandmill as defined in claim 3 in which:

f. the upper end (61) of the shear blade is dowelled in an upper anchorplate (73) by dowels (75), the anchor plate being mounted on a firstpivot having an axis (79),

g. the lower end (63) of the shear blade is dowelled in a lower anchorplate (74) by dowels (76), the lower anchor plate being mounted on asecond pivot having an axis (81), one pivot being moveable relative tothe other pivot for applying a tensile load to the shear blade.

6. A bandmill according to claim including:

h. a pivot clevis (1 16), the clevis having a stud (121) extendingco-axially with an axis (83) adjoining the pivot axes,

i. a pin (118) concentric with the first pivot axis and secured to theclevis,

j. a portion (112) of the bandmill having a bore (1 14), adapted tosupport the stud,

k. a nut (25) being threaded on the stud,

l. measuring means cooperating with the nut, stud and the portion of thebandmill to measure load applied to the shear blade, constructed andarranged so that tightening of the nut applies a load to the shearblade, indication of load being read on the measuring means.

7. Structure according to claim 6 in which the measuring means tomeasure load applied to the shear blade includes:

m. a stack of load indicating washers (124) interposed between the nut(125) and the portion (112) of the bandmill, so that tightening of thenut on the stud applies a load to the shear blade and deflects the stackof washers, deflection being an indication of load applied to the shearblade.

8. Structure as defined in claim 1 wherein the shear blade is a sawsteel having an ultimate tensile strength range from about 200,000 to210,000 pounds per square inch and a yield strength of 180,000 to190,000 pounds per square inch.

9. Structure as defined in claim 2 wherein the shear blade is a sawsteel having an ultimate tensile strength range from about 200,000 to210,000 pounds per square inch and a yield strength of 180,000 to190,000 pounds per square inch.

1. A bandmill (10) having a band saw (14) adapted to cut a log (22)moving in a direction on a conveyor (23), the saw having a thickness(32) and a cutting portion (24), the cutting portion having a planeparallel with the direction of movement of logs, the saw being adaptedto cut a kerf (30) in the log, the kerf having a width (31), thebandmill including: a. a shear blade (16) mounted coplanar with andspaced behind the cutting portion of the saw, the shear blade havingleading and trailing edges (66, 67) a center portion (98), and athickness (33) less than the width of the kerf, b. tensioning means totension the shear blade so that the leading edge is at a higher tensilestress than the center portion of the shear blade adapted so thatmovement of the logs through the saw is essentially axial translationwith negligible rotation of the log.
 2. A bandmill as defined in claim 1in which the tensioning means to tension the shear blade includes: c.tire portions (102, 104) at the leading and trailing edges of the shearblade, the tire portions being produced by roll tensioning, adapted sothat, when the shear blade is subjected to tensile stress, the tireportions are at a higher tensile stress than the center portion (98) ofthe shear blade.
 3. A bandmill as defined in claim 1 in which the shearblade has mid-points (68, 69) at upper and lower ends (61, 63), and thetensioning means to tension the shear blade includes: d. pivots at theupper and lower ends of the shear blade the pivots having axes (79, 81),at least one axis of a pivot being displaced forward of a mid-point ofone of the ends of the shear blade, the pivot points being adapted sothat a tensile load applied between the pivot points is applied to theshear blade, adapted so that, when a tensile load is applied to thepivots, the leading edge of the shear blade is at a higher tensilestress than the trailing edge and center portion.
 4. A bandmill asdefined in claim 1 in which: e. the shear blade is a quadrilateral withthe trailing edge longer than the leading edge, so that when the tensileload is applied to the shear blade the leading edge is at a highertensile stress than the trailing edge.
 5. A bandmill as defined in claim3 in which: f. the upper end (61) of the shear blade is dowelled in anupper anchor plate (73) by dowels (75), the anchor plate being mountedon a first pivot having an axis (79), g. the lower end (63) of the shearblade is dowelled in a lower anchor plate (74) by doweLs (76), the loweranchor plate being mounted on a second pivot having an axis (81), onepivot being moveable relative to the other pivot for applying a tensileload to the shear blade.
 6. A bandmill according to claim 5 including:h. a pivot clevis (116), the clevis having a stud (121) extendingco-axially with an axis (83) adjoining the pivot axes, i. a pin (118)concentric with the first pivot axis and secured to the clevis, j. aportion (112) of the bandmill having a bore (114), adapted to supportthe stud, k. a nut (25) being threaded on the stud, l. measuring meansco-operating with the nut, stud and the portion of the bandmill tomeasure load applied to the shear blade, constructed and arranged sothat tightening of the nut applies a load to the shear blade, indicationof load being read on the measuring means.
 7. Structure according toclaim 6 in which the measuring means to measure load applied to theshear blade includes: m. a stack of load indicating washers (124)interposed between the nut (125) and the portion (112) of the bandmill,so that tightening of the nut on the stud applies a load to the shearblade and deflects the stack of washers, deflection being an indicationof load applied to the shear blade.
 8. Structure as defined in claim 1wherein the shear blade is a saw steel having an ultimate tensilestrength range from about 200,000 to 210,000 pounds per square inch anda yield strength of 180,000 to 190,000 pounds per square inch. 9.Structure as defined in claim 2 wherein the shear blade is a saw steelhaving an ultimate tensile strength range from about 200,000 to 210,000pounds per square inch and a yield strength of 180,000 to 190,000 poundsper square inch.